The full-load, full-speed testing of internal combustion engines for indicated power, brake power and friction power can be accomplished by connecting the engine to a motoring dynamometer capable of absorbing the full-load output of the engine and capable of driving the maximum frictional load of the engine while measuring speed and torque. Such dynamometers are very large, very expensive, and very inconvenient to use.
A much more economical and convenient apparatus for testing spark-ignition internal combustion engines under simulated full-load conditions is apparatus for operating the engine with all but one of n ignitions interrupted, where n may be greater than the number of cylinders, so that all cylinders are operated in sequence under full power conditions. The engine operates at full speed driving frictional and pumping loads, without danger of overspeeding and damaging the engine.
An economical and convenient brake power test of compression-ignition internal combustion engines under simulated full-load conditions is the acceleration burst test in which an engine initially operating at idle speed is suddenly given full throttle and caused to accelerate to a maximum governed speed. The inertia of the engine is the load on the engine, and the time taken to accelerate through a low speed to a high speed is a measure of the full-power output capability of the engine. This test is particularly useful for testing diesel engines, and provides a somewhat less accurate indication of the condition of a spark-ignition engine equipped with a carburetor instead of fuel injectors.
The accuracy with which the full output power capability of the engine is given by the time taken to accelerate from a low speed to a high speed depends on the accuracy of the speed measurements, which are in units of angular displacement (such as revolutions) divided by units of time (such as minutes or seconds). An accurate measurement of speed is complicated by the fact that the burst acceleration from a low speed to a high speed takes only about one second or less. Another complication has been found to be due to pulsations in instantaneous speed which are due to explosions and compressions in individual cylinders of the engine. The pulsations in instantaneous speed are particularly disturbing if they are non-uniform due to malfunctioning of one or more individual cylinders of the engine.
An additional problem associated with this type of test is that of interpreting the results. A low output power indication may be caused by relatively high power absorption of the engine and its accessories. Large variations in frictional load can be expected from test to test on different vehicles in different conditions with different accessory loads since the frictional load on an engine is dependent on oil type and temperature, accessory load, and other effects. Thus, a very significant engine power test parameter is the indicated (total) power developed within the engine. Knowing this and the brake (output) power capability of an engine allows for differentiation between internal and external engine problems, and sometimes differentiation between real and apparent engine problems.